1. Field of the Invention
This invention relates to a test device for testing the effectiveness of ozone-ultraviolet cleaning-stripping equipment and a method for its use.
2. Background Information
During the manufacture of silicon-based electronic devices and like workpieces, there are times when it is necessary to strip away organic layers such as photoresist layers or to remove organic contamination from a surface of the workpiece. Oxidative environments have been used for this purpose.
The oxidation power of an environment containing a mixture of ozone and oxygen in the presence of short-wavelength ultraviolet (UV) radiation for cleaning and stripping organic compounds from surfaces or surface treatment has already been demonstrated (1). (J. R. VIG, Treatise on Clean Surface Technology, Vol 1 (New York, N.Y.: Plenum Press, 1987), p. 1-22.)
It is generally thought that the combination of UV light and ozone destroys organic compounds by photosensitized oxidation. The organic molecules are excited, dissociated, or depolymerized by short-wavelength UV light, such as that emitted by a low-pressure mercury lamp in a quartz envelope. These excited molecules are particularly likely to undergo oxidation by atomic oxygen, ozone or other minor oxidants. One primary reaction thought to occur when ozone and oxygen molecules are exposed to UV radiation from a mercury lamp is the conversion of oxygen to ozone. At elevated temperatures a second reaction occurs. Ozone thermally decomposes in the gas phase forming atomic and molecular oxygen. A. E. AXWORTHY, JR. AND S. W. BENSON, Advances in Chemistry Series, No.21 (Washington, D.C.: American Chemical Society, 1959),p.383-397. Atomic oxygen is a very effective oxidizer.
The primary UV/ozone oxidation products from most organic compounds are the volatiles, water and carbon dioxide. An auxiliary ozone generator is often added to UV/ozone cleaning systems to further increase the concentration of oxidants and thereby shorten the process time. It has been shown that an elevated temperature also accelerates the rate of oxidation of organic matter when using UV/ozone. (0. TSUJI, T. TATSUTA AND K. DEGUCHI, "Instrumentation for Photoresist Stripping by Combined System of Silent Discharge Ozone with UV-1 Radiation", (Proceedings of the International Symposium On Plasma Chemistry, Eindhoven, Netherlands, 1985), p.1055-1060; P. C. WOOD, T. WYDEVEN, AND 0. TSUJI, "Critical Process Variables for UV-1 Ozone Etching of Photoresist", (Materials Research Society Proceedings, San Francisco, Calif., 1993), p.237-242. For example, at an ozone/oxygen flow rate of 0.5 liters per minute (1-min.sup.-1) and an ozone concentration of 6.4 grams per cubic meter (g-m.sup.3), the rate of oxidation of a photoresist at 150.degree. C. was only 54 Angstroms per minute while at 300.degree. C. it was 596 Angstroms per minute, i.e., 10 times faster.
The equipment used to carry out these strippings and cleanings includes a base or stage upon which the workpiece is positioned, a source of ultraviolet light focused on the workpiece, and an ozone source capable of flowing a mixture of ozone and oxygen over the work piece. It is important that this equipment operate efficiently with a uniform degree of oxidation across the entire surface of the workpiece. It would be desirable to have a way to monitor the operation of this equipment and the effectiveness of the oxidation process.